It's not just the $70 billion; it's not just the added costs of health care, pensions, etc. over the next 20 years, either. It's not even all those and the added debt, and therefore the interest costs.
The costs of more troops also involves what we commit ourselves to do in the next 20 years, and the costs of what we therefore have to forego.
Like better healthcare for the rest of us, like better education, etc.
But also, the 90,000 odd troops will be less qualified, more unhealthy, less reliable, and include more foreigners, because the army, especially, has been unable to meet its recruitment quotas even with the smaller existing army.
Further, worst of all, the cost will be our committment to continue to "solve problems" by military means, whether it is fighting terrorism, or ensuring adequate energy. Now, we fight in both Iraq and Afghanistan, while bin Laden is nowhere to be seen, and while more and more terrorists are able to train in the chaos caused by our troops in Iraq, and to be recruited because of the growing hatred of US strong-arm tactics in the Muslim world.
The US would be much better off with a smaller military and a diminished ability to intervene in the rest of the world; it would be much better off if the UN was therefore forced to deal with world problems, and the US was only a prominent contributor to helping to resolve them.